Fiber-optic communication networks serve a key demand of the information age by providing high-speed data between network nodes. Fiber optic communication networks include an aggregation of interconnected fiber-optic links. Simply stated, a fiber-optic link involves an optical signal source that emits information in the form of light into an optical fiber. Due to principles of internal reflection, the optical signal propagates through the optical fiber until it is eventually received into an optical signal receiver. If the fiber-optic link is bi-directional, information may be optically communicated in reverse typically using a separate optical fiber.
Fiber-optic networks are used in a wide variety of applications, each requiring different lengths of fiber-optic links. For instance, relatively short fiber-optic links may be used to communicate information between a computer and its proximate peripherals, or between local video source (such as a DVD or DVR) and a television. On the opposite extreme, however, fiber-optic links may extend thousands of kilometers when the information is to be communicated across the globe. For instance, a submarine fiber-optic link may rest on an ocean floor spanning entire oceans to thereby connect two remote continents.
Transmission of optic signals over such long distances presents enormous technical challenges. Significant time and resources may be required for any improvement in the art of submarine and other long-haul optical communication. Each improvement can represent a significant advance since such improvements often lead to the more widespread availability of communication throughout the globe. Thus, such advances may potentially accelerate humankind's ability to collaborate, learn, do business, and the like, regardless of where an individual resides on the globe.
Conventionally, installed submarine systems are designed to employ Dense Wavelength Division Multiplexing (DWDM) in which information is communicated over N channels (where N is an integer that is often 16 or more), each channel corresponding to a particular wavelength. Conventional installed submarine fiber-optic links include N channels of 2.5 gigabits per second (Gbit/s) or N channels of 10 Gbit/s data, and use Amplitude Shift Keying (ASK) (also called On-Off-Keying (OOK)) modulation. At 10 Gbit/s, such channels might be separated by, for example, 100 gigahertz (GHz), 50 GHz, or even smaller provided that inter-channel interference does not begin to degrade the signal.
Submarine fiber-optic links use single-mode fiber in which the primary dispersion mechanism is called “chromatic dispersion” (often also called “material dispersion”). This chromatic dispersion occurs because optics of different wavelengths tend to travel through the optical fiber at slightly different speeds. Without adequate compensation, this can result in the distortion and eventual loss of the signal over the long length of the optical fiber.
Some optical fibers are “positive dispersion” fiber in which the longer wavelength (lower frequency) light travels through the fiber slightly slower than the shorter wavelength (higher frequency) light. Other optical fibers are “negative dispersion” fiber in which the longer wavelength (lower frequency) light travels through the fiber slightly faster than the shorter wavelength (higher frequency) light. By mixing the use of negative dispersion and positive dispersion fibers, the dispersion can often be largely (but often not completely) cancelled out.
Submarine fiber-optic links remain sensitive to this portion of dispersion that is not cancelled out through the mixing of positive and negative dispersion fibers. Accordingly, conventional submarine fiber-optic systems often employ post-compensation of the chromatic dispersion or optimize the post-compensation only even if some pre-compensation is applied to obtain best performance.
Conventional submarine systems widely use a mix of Standard Single Mode Fiber (SSMF) and Non-Zero Dispersion Shifted Fiber (NZDSF), which results in a particular dispersion map as the accumulated dispersion is tracked across the length of the fiber for different wavelength channels.
Differential Phase Shift Keying (DPSK) modulating is a modulation mechanism that has been shown to present an approximate 3 decibel (dB) improved noise performance over ASK. However, the application of DPSK to submarine systems that have this kind of dispersion map is not at all straightforward. For instance, it has been found that the performance of 10 Gbit/s return-to-zero DPSK (RZ-DPSK) is significantly degraded for wavelengths near the accumulated “dispersion zero” region of the NZDSF fiber where the dispersion is regularly well compensated for along the system length. However, at the longer and shorter wavelength channels towards edges of the system gain bandwidth (where the dispersion slope leads to dispersion accumulation along the line and bit-overlapped transmission), the performance of RZ-DPSK showed the expected improvement over ASK.
This degraded performance near the “dispersion zero” region has been attributed to stronger Kerr-effect based interactions which lead to a nonlinear phase noise which increases the bit error rate. It has been shown that not only Self Phase Modulation (SPM) but also cross (X) Phase Modulation (XPM) can lead to such degradation—particularly for low bitrates of 10 Gbit/s and narrow channel spacing (<50 GHz).
One potential solution to this problem is to replace the degraded DPSK central channels by some with Return to Zero ASK (RZ-ASK) modulation, which performs best when there is low accumulated dispersion as in the “zero dispersion” region.